1. Field of the Invention
The present invention relates generally to drag reducing methods for low molecular weight fluids. More specifically, the present invention relates to separation of drag reducers from low molecular weight liquids, such as hydrocarbons and anhydrous ammonia.
2. Description of the Prior Art
When fluids are transported by a pipeline, a drop in fluid pressure typically occurs due to friction between the wall of the pipeline and the fluid. Due to this pressure drop, for a given pipeline, fluid must be transported with sufficient pressure to achieve a desired throughput. When higher flow rates are desired through the pipeline, more pressure must be applied due to the fact that as flow rates are increased the difference in pressure caused by the pressure drop also increases. However, design limitations on pipelines limit the amount of pressure that can be employed. The problems associated with pressure drop are most acute when fluids are transported over long distances. Such pressure drops can result in inefficiencies that increase equipment and operation costs.
To alleviate the problems associated with pressure drop, many in the industry utilize drag reducing additives in the flowing fluid. When the flow of fluid in a pipeline is turbulent, high molecular weight polymeric drag reducers can be employed to enhance the flow. A drag reducer is a composition capable of substantially reducing friction loss associated with the turbulent flow of fluid through a pipeline. The role of these additives is to suppress the growth of turbulent eddies, which results in higher flow rate at a constant pumping pressure. Ultra-high molecular weight polymers are known to function well as drag reducers, particularly in hydrocarbon liquids. In general, drag reduction depends in part upon the molecular weight of the polymer additive and its ability to dissolve in the hydrocarbon under turbulent flow. Effective drag reducing polymers typically have molecular weights in excess of five million.
Low molecular weight hydrocarbon fluids such as natural gas liquids (“NGLs”) and liquefied petroleum gases (“LPGs”) often are pipelined long distances under pressure as liquids. NGLs are often fractionated into their components which can include, but are not limited to, methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12), and heavier cuts (C6+). LPGs can comprise fractionated ethane, propane, and butane that result from the processing of NGLs. Other LPGs can include, but are not limited to, iso-butane, dimethyl ether (CH3OCH3), and natural gasoline (or condensate). Ethylene which results from the cracking of ethane is also transported as an LPG.
Liquefied natural gas (“LNG”) is a natural gas that has been cooled to below its boiling point (about −260° C.) such that it condenses to a liquid. In LNG applications, the liquid is at temperatures where drag reducing agents (“DRAs”) do not dissolve readily and, thus, usually cannot provide drag reduction. In general, most LNG applications do not require drag reducers since the LNG typically can be transported over long distances as cargo in insulated ships or trucks. Pumping as a liquid generally only occurs during loading and unloading of these vessels.
NGLs or LPGs generally are conducive to high levels of drag reduction, because when they are being transported in pipelines, the fluid is in turbulent flow and because the hydrocarbon DRA polymers are very soluble in the hydrocarbon fluid. However, typically, DRAs have not been utilized to drag reduce NGLs or LPGs due to a vast difference in the volatility of the DRA polymers relative to the NGL or LPG components.
The active ingredient in most commercial hydrocarbon DRAs for pipelines is an ultra-high molecular weight poly-alpha-olefin polymer. Because of the extremely large molecular size, the polymer molecules usually do not vaporize at any reasonable temperature. In fact, because of the large molecular size, the poly-alpha-olefin polymer can begin to thermally degrade at temperatures (about 550° F.) well below any theoretical polymer vaporization temperature. At temperatures above about 550° F., the polymer can break down into much smaller components which can vaporize at reasonable temperatures. Likewise, carrier fluids and other inactive components of many DRAs are larger molecules which do not vaporize unless temperatures are greater than about 360° F. However, in many cases, NGLs eventually can be vaporized in processing at temperatures well below about 360° F. or even about 550° F. If lower temperature vaporization occurs, usually only the NGL components will vaporize, and the DRA components will remain behind at the location of the vaporization.
If the NGL is only partially vaporized, then the DRA components can remain in solution, i.e., in the remaining unvaporized (liquid) portion of the NGL. The DRA then can be carried through the process in solution in this liquid portion, albeit now at a higher concentration due to the lower amount of liquid present after the partial vaporization. If full vaporization of the NGL occurs at a low temperature, then the DRA can remain behind at the location of full vaporization and, in theory, will have no liquid NGL component to carry it any further through the process. The DRA which deposits or remains behind at this location can be in a rubbery, semi-solid form, possibly along with some liquid portions of a DRA carrier. These dynamics can be present in the distillation/fractionation processes for NGL and are especially present in reboiler units. Depending upon the design of the reboiler unit, partial or full vaporization of the hydrocarbon stream can occur, and periodic flushes can be required to prevent over-concentration or deposition of DRA within the reboiler unit.
For the end use of LPGs (usually butane and propane) treated with DRAs, similar downstream effects need to be considered. If the LPG is used for blending into other liquid streams, such as fuels, then the presence of the DRA will, most likely, not be an issue. For instance, butane can be blended into gasoline, which is already an automobile fuel which is often treated with DRA. If the LPG is to be used in a downstream chemical process or directly as a fuel, then that process or end-use needs to be examined in detail for potential points of full vaporization. Fuels, such as propane, typically can be vaporized prior to carburization, and any DRA can “fall out” in the vaporizer unit.
Polymer deposition or “fallout” can cause problems if deposition occurs in an undesired location. Since poly-alpha-olefin DRA polymers can be soluble in hydrocarbon (e.g. oil, diesel fuel, kerosene), the polymer possibly can be solvated and flushed from the system. However, depending upon the amount of fallout and consistency of the deposited material, the solvation and flushing process can take considerable time, considerable volumes of solvent, and likely can require equipment downtime. Because DRAs can be soluble in the liquid medium they are drag reducing, DRA polymers cannot be removed by simple methods such as filtration. Therefore, a method to easily remove DRA polymers from low molecular weight pipeline liquids is needed.